Persian Conquest

Assyria and Babylonia

Leaving Egypt in 331 BC, Alexander marched eastward into Mesopotamia (now northern Iraq) and again defeated Darius, at the Battle of Gaugamela. Darius once more fled the field, and Alexander chased him as far as Arbela. Gaugamela would be the final and decisive encounter between the two. Darius fled over the mountains to Ecbatana (modern Hamedan), while Alexander captured Babylon.

Persia

From Babylon, Alexander went to Susa, one of the Achaemenid capitals, and captured its treasury. He sent the bulk of his army to the Persian ceremonial capital of Persepolis via the Persian Royal Road. Alexander himself took selected troops on the direct route to the city. He then stormed the pass of the Persian Gates (in the modern Zagros Mountains) which had been blocked by a Persian army under Ariobarzanes and then hurried to Persepolis before its garrison could loot the treasury.

On entering Persepolis, Alexander allowed his troops to loot the city for several days. Alexander stayed in Persepolis for five months. During his stay a fire broke out in the eastern palace of Xerxes I and spread to the rest of the city. Possible causes include a drunken accident or deliberate revenge for the burning of the Acropolis of Athens during the Second Persian War by Xerxes; Plutarch and Diodorus allege that Alexander's companion, the hetaera Thaïs, instigated and started the fire. Even as he watched the city burn, Alexander immediately began to regret his decision. Plutarch claims that he ordered his men to put out the fires, but that the flames had already spread to most of the city. Curtius claims that Alexander did not regret his decision until the next morning. Plutarch recounts an anecdote in which Alexander pauses and talks to a fallen statue of Xerxes as if it were a live person: Shall I pass by and leave you lying there because of the expeditions you led against Greece, or shall I set you up again because of your magnanimity and your virtues in other respects?

Fall of the Empire and the East

Alexander then chased Darius, first into Media, and then Parthia. The Persian king no longer controlled his own destiny, and was taken prisoner by Bessus, his Bactrian satrap and kinsman. As Alexander approached, Bessus had his men fatally stab the Great King and then declared himself Darius' successor as Artaxerxes V, before retreating into Central Asia to launch a guerrilla campaign against Alexander. Alexander buried Darius' remains next to his Achaemenid predecessors in a regal funeral. He claimed that, while dying, Darius had named him as his successor to the Achaemenid throne. The Achaemenid Empire is normally considered to have fallen with Darius.

Alexander viewed Bessus as a usurper and set out to defeat him. This campaign, initially against Bessus, turned into a grand tour of central Asia. Alexander founded a series of new cities, all called Alexandria, including modern Kandahar in Afghanistan, and Alexandria Eschate ("The Furthest") in modern Tajikistan. The campaign took Alexander through Media, Parthia, Aria (West Afghanistan), Drangiana, Arachosia (South and Central Afghanistan), Bactria (North and Central Afghanistan), and Scythia.

In 329 BC, Spitamenes, who held an undefined position in the satrapy of Sogdiana, betrayed Bessus to Ptolemy, one of Alexander's trusted companions, and Bessus was executed. However, when, at some point later, Alexander was on the Jaxartes dealing with an incursion by a horse nomad army, Spitamenes raised Sogdiana in revolt. Alexander personally defeated the Scythians at the Battle of Jaxartes and immediately launched a campaign against Spitamenes, defeating him in the Battle of Gabai. After the defeat, Spitamenes was killed by his own men, who then sued for peace.